1. Field of the Invention
The present invention relates in general to a cathode ray tube, and more particularly, to a stray emission prevention circuit for a cathode ray tube.
2. Description of the Prior Art
It is well known in the art that a cathode ray tube is an essential component in a television receiver or a monitor.
Construction and operation of a conventional cathode ray tube will hereinafter be described briefly with reference to FIGS. 1 and 2.
FIG. 1 is a view illustrating the construction of the conventional cathode ray tube. The cathode ray tube comprises a neck 13 and a convergence device 7 disposed in the neck 13, which performs fine convergence alignment of paths of three electron beams corresponding respectively to red (R), green (G) and blue (B) color signals. An electron gun 10 is also provided to scan the three electron beams therefrom, the paths of which are finely aligned.
The convergence device 7 consists of a static convergence permanent magnet for aligning the convergence of the electron beams at the center of a screen and a dynamic convergence coil for aligning the convergence of the electron beams at the peripheral portion of the screen. The convergence device 7 allows the three electron beams to simultaneously pass through corresponding apertures of a shadow mask 3 over the whole of the screen, the shadow mask 3 being fixed to a frame 11.
The three electron beams are then deflected at a desired angle in every direction by a deflection yoke 6 which is fixedly attached to a funnel 2. The deflection yoke 6 determines the deflection direction and deflection angle of the electron beams in response to a signal which is applied thereto from the outside, such as, for example, a saw tooth wave signal. After passing through the corresponding slots of the mask 3 fixed to the frame 11, the deflected electron beams impinge on phosphors 4 which are coated on the inner surface of a panel 1, thereby to bring out the corresponding colors. In other words, the three electron beams simultaneously pass through the corresponding apertures of the shadow mask 3 at the deflection angle and then impinge on the phosphors 4 at the individual different angles since they pass through the corresponding apertures in the individual different directions. Because red, green and blue color phosphors constitute a unit phosphor, the impinging three electron beams bring out various colors at the corresponding phosphor positions on the screen in accordance with the impinging directions and intensities thereof. For this reason, the shadow mask 3 may be referred to as a dichroic electrode.
Also provided on the upper portion of the funnel 2 is an anode cap 12 to which an anode voltage is applied as a drive voltage to the cathode ray tube. Upon application of a high voltage of about 30 KV or the anode voltage to the anode cap 12, the applied high voltage causes the three electron beams scanned from the electron gun 10 to be strongly sucked toward the panel 1 and then to impinge on the phosphors 4, since the three electron beams are of negative electrons.
The high voltage of about 30 KV through the anode cap 12 is applied to an anode electrode G4 of the electron gun 10 through a graphite and a getter 8, which are coated on the inner surface of the cathode ray tube. The getter 8 is disposed in the cathode ray tube for the purpose of increasing vacuum level in the cathode ray tube. The getter 8 has about 300 mg of barium, which absorbs gas molecules such as, for example, hydrogen, nitrogen, carbon dioxide, carbon and etc. in the cathode ray tube, thereby to increase the vacuum level in the cathode ray tube. As mentioned above, the getter 8 acts to apply therethrough to the anode electrode G4 the high voltage of 30 KV which is applied through the anode cap 12.
Unlike the voltage to the anode electrode G4, voltages to the other electrodes in the electron gun 10, i.e., control grid G1, cathode K, screen grid G2 and focus grid G3 and to a heater H are separately applied through corresponding terminals.
The voltages necessary to the respective electrodes in the actual operation are as follows:
cathode K: 100-180 V PA1 control grid G1: 0-100 V PA1 screen grid G2: 300-700 V PA1 focus grid G3: 4-6 KV PA1 anode G4: 20-30 KV
On the other hand, an inner shield 5 is fixedly attached to the frame 11 in the cathode ray tube in order to prevent the scanned electron beams from being influenced by a terrestrial magnetism. The inner shield 5 acts to screen the terrestrial magnetism, thereby to avoid degradation of color purity.
Noticeably, stray electrons may be generated in the conventional cathode ray tube because of alien substances incoming during a manufacturing process of the cathode ray tube and stuck on the components of the cathode ray tube. In other words, the alien substances react with the residual high voltage of the anode electrode G4 which may be present upon power-off of the television receiver or monitor, resulting in generation of the stray electrons. The generated stray electrons then impinge on the phosphors 4. This results in unnecessary luminescence of the phosphors 4. This phenomenon is called stray emission.
In accordance with the manufacturing process of the conventional cathode ray tube, the alien substances are burned up by carrying out a knocking process in an aging process. This is performed for the purpose of preventing the generation of the stray electrons although the residual high voltage of the anode electrode G4 is present upon power-off of the television receiver or monitor. The aging process is applied for activation of thermionic emission from the electron gun. Namely, joule heat is generated by applying the voltage to the heater H to heat it and then applied to the cathode K layer of oxide, thereby causing the thermions (negative electrons) to be emitted from the cathode K layer. The emitted thermions are focused and accelerated by the electrodes of the electron gun 10 and then arrive at the phosphors 4.
Also, the knocking process applies the high voltage into the cathode ray tube to burn up the alien substances.
However, the conventional cathode ray tube has a disadvantage as follows. For the purpose of complete removal of the alien substances in the cathode ray tube by the knocking process, it is necessary to apply a very high voltage into the cathode ray tube. There is, however, a limit to raising the high voltage. That is, raising the voltage very high causes a discharge between magnetic poles (not shown) of the electron gun 10, which has a bad affect on the cathode K. For this reason, in the manufacturing process of the cathode ray tube, a proper high voltage must be considered for the knocking process as well as to remove the alien substances in the cathode ray tube. In result, some of the alien substances may remain in the cathode ray tube and react with the residual high voltage which is present upon power-off of the television receiver or monitor, resulting in the stray emission phenomenon on the screen.
FIG. 2 is a graph illustrating a natural discharge of the high voltage of 30 KV applied to the anode cap in the cathode ray tube is with the lapse of time after power-off of a system having the cathode ray tube, such as the television receiver or monitor. From this figure, it can be seen that it takes the high voltage of 30 KV a long time to naturally discharge completely.